1. Field of the Invention
The present invention relates to a printed circuit board, more specifically to a flexible printed circuit board including a reinforcement board and a manufacturing method thereof.
2. Description of the Related Art
Recently, because of miniaturization of elements and reduction in the size of electronic devices, such as portable telephones and digital cameras, the interior space in these electronic devices has decreased. Therefore, it is difficult to provide electronic elements, such as a printed circuit board or a wiring board in such a small space.
Generally, a flexible board, such as a FPC (a flexible printed circuit board) and an FPWB (a flexible printed wiring board) is used widely as a suitable solution for such a narrow, small space. The FPC may be modified with a reinforcement board attached at, for example, a connector part or an element mounting part of the board in order to increase mechanical integrity and to increase the rigidity in the portions.
FIGS. 1A and 1B (prior art 1) and 2A-2C (prior art 2) illustrate the technology of a FPC with the reinforcement board in prior art.
In the FPC shown in FIG. 1A, a wiring layer 22 is formed by patterning a copper foil adhered to a insulated substrate 21 made of a polyimide resin. The sections of reinforcement board 23a and 23b are formed correspond to the reinforcement portions X1 and X2 in the wiring layer 22, as a connector part which needs to be reinforced. The reinforcement boards 23a and 23b are arranged to face the adhesive layers 24a and 24b provided on a face of each reinforcement board 23a and 23b in the direction of an undersurface of the insulated substrate 21.
Positioning holes 25a1 and 25b1 in the insulated substrate 21 and the positioning holes 25a2 and 25b2 in the reinforcement boards 23a and 23b overlap with each other to receive the guide pins 26a and 26b, respectively. Then, the reinforcement boards 23a and 23b are positioned to face the reinforcement portions X1 and X2. As shown in FIG. 1B, the reinforcement boards 23a and 23b are pressed towards the undersurface of the insulated substrate 21 under high temperature and high-pressure conditions, and secured thereto by the adhesives layers 24a and 24b. In this case, the guide pins 26a and 26b are removed. The technique to attach the sections of reinforcement board to the insulated substrates by the adhesive layers is described in Japanese Patent Laid-Open Publication No. 2002-314207.
A technique developed by the present inventor is shown in FIG. 2A. A reinforcement substrate 33 is adhered by thermo-compression bonding, for example, to an undersurface of a polyimide resin insulated substrate 31 that has a copper foil 32a on the upper surface. A wiring layer 32 is formed by patterning the copper foil 32a as shown in FIG. 2B. As shown in FIG. 2C, the photo-mask layers 34 and 35 are formed on an undersurface of the reinforcement substrate 33, corresponding to the reinforcement portions X1 and X2, so as to increase structural integrity. The reinforcement plates 33a and 33b are formed by chemical etching on the exposed portion of the reinforcement substrate 33.
The insulated protection layers 27 and 36 on the wiring layers 22 and 32, such as a solder resist, are shown in FIG. 1A and FIG. 2C, respectively, and the deformable portion is indicated as Y.
However, according to the prior art 1 shown in FIG. 1A, the adhesives layers 24a and 24b spread along the undersurface of the insulated substrate 21 through the side of the reinforcement plates 23a and 23b, due to applied pressure at the time of adhesion. The overflow portions 29a and 29b are generated due to the applied pressure, which may adhere unnecessary materials that prevent the deformation of the insulated substrate 21.
Moreover, positioning of the reinforcement plates 23a and 23b requires the positioning holes and the guide pins so that the manufacturing operation is complicated. There is a problem that positioning accuracy is as low as about ±100 micrometers positioning gap. As the printed circuit board becomes smaller, the problem becomes more serious. When the positioning is performed by visual adjustment, the accuracy is further decreased.
As shown in FIG. 2C, the positioning accuracy is improved in the prior art 2 since it is based on photolithography technology. But, since the etching near the insulated substrate 31 is slow, a taper etching of the side wall of the reinforcement plates 33a and 33b is carried out and the skirt portions 36a and 36b of each taper, which approaches and extends toward the insulated substrate 31 may form a sharp edge. Therefore, when the insulated substrate 31 is twisted and bent on an apparatus, the insulated substrate 31 may be damaged and develop a fracture in response to the concentration of stress at one point.